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Preliminary number of tornadoes this year as reported by the NOAA/Storm Prediction Center

OverviewThe “official” Atlantic Basin hurricane season has just begun and it may be more active than recent years and also compared to normal (tropical outlook), but so far the tornado season in the US has been below-normal which continues a trend over the past several years. Through the end of May, the preliminary number of tornadoes reported across the US is 605 and this compares to the 10-year normal of 788. This below-normal reading at the end of May comes despite a quick start to above-normal levels early in the year following a somewhat unusual large outbreak of tornadoes during the month of February. Fortunately, it has not been since 2011 in which the US actually had an above-normal number of tornadoes at this time of year.

Year-by-year comparison back to 2005 of the number of tornadoes through the month of May; courtesy NOAA/SPC

Some reasons for the below-normal activityIn a typical tornado season, favorable conditions for widespread outbreaks in the central Plains and Midwest generally include colder-than-normal conditions to the north and west (i.e., across the Rockies, Pacific Northwest) and hot, humid conditions to the south and east (i.e., in the Deep South). However, during the months of March, April and May of this year, this type of "favorable" temperature pattern for tornadoes was simply not too prevalent. In fact, the composite map for April temperature anomalies (bottom, left) across the US shows warmer-than-normal conditions across the Northern Rockies/Pacific Northwest and generally near normal temperatures in the Deep South. In addition, the precipitation anomalies map for April (below, right) shows that the Deep South and central Plains were wetter-than-normal and this actually helped to keep temperatures down somewhat compared to what could have occurred if soil moisture was low.